1. Field of the Invention
The present invention relates to a coherent light source apparatus such as a variable wavelength semiconductor laser for use in a lightwave interferometer that obtains various mechanical measures. More particularly, the present invention relates to a light source apparatus having a wavelength-stabilized output light.
2. Description of the Related Art
A semiconductor laser is a coherent light source, which varies its oscillation wavelength in accordance with a temperature and an injection current. This wavelength variability of the semiconductor laser shows many possibilities in the field of the lightwave interferometry. For example, when plural light sources with slightly different wavelengths are required, a plurality of lasers must be employed if they are stabilized lasers of fixed oscillation wavelength type, while only a single semiconductor laser of variable wavelength type can respond to this case by switching its output wavelength.
Known wavelength stabilization technologies for the semiconductor laser include a temperature control method, an injection current control method, an elementary-ray absorption method and a method that employs an external resonator together. The temperature control method, however, needs much time for switching of temperatures and can not perform fast switching of output wavelengths. The temperature control and injection current control methods can not ensure the oscillation wavelength to be set accurately to a desired value even if a temperature or injection current is controlled to a predetermined setting. The elementary-ray absorption method, even though it can achieve wavelength stabilization, is not adaptive to a variable wavelength light source in a measurement system that requires an arbitrary switching of wavelengths. In the method that employs the external resonator, its light source structure becomes complicated.
The present invention has been made in consideration of the above and accordingly has an object to provide a wavelength-stabilized light source apparatus capable of performing fast wavelength stabilizing control with a designation of an arbitrary wavelength.
First, the present invention is provided with a wavelength-stabilized light source apparatus, which comprises a coherent light source of which wavelength is variably controlled by a driver; a beam splitter for splitting the output light from the coherent light source to obtain a part thereof as a control light; a interference optical system for further splitting the control light is two, then giving a predetermined optical path length difference to the resultant two control lights and synthesizing them to generate interference fringes; a modulator for giving a variation to the optical path length difference in the interference optical system to modulate the interference fringes to be obtained; a photoreceptive device for receiving the interference fringes obtained from the interference optical system; a demodulator for detecting a degree of modulation of a received signal obtained from the photoreceptive device; and a controller for feedback controlling a driving condition for the driver so that the output from the demodulator matches to a predetermined setting.
According to the first wavelength-stabilized light source apparatus, a part of the output light from the coherent light source is extracted as the control light, which enters the interference optical system to modulate the optical path length difference, resulting in the interference fringes. Using the dependency of the degree of modulation of the received signal on the wavelength, the wavelength stabilization is performed by feedback controlling the driver so that the demodulated output matches to the setting. Thus, by designation of an arbitrary wavelength from the external, the output wavelength of the coherent light source can be stabilized at a high speed with a high precision.
In the first wavelength-stabilized light source apparatus, the modulators are arranged, for example, respectively on the optical paths for the two control lights in the interference optical system. The modulators includes two acoustic optical modulators (AOM) driven with different angular frequencies to generate an interfered beat with a difference between the angular frequencies, xcfx89c; and an electrical optical modulator (EOM) arranged on the optical path for one of the two control lights in the interference optical system to phase-modulate the output light in the from of a sinusoidal wave with an angular frequency of xcfx89. The demodulator includes a phase detector for FM-demodulating the received signal using a carrier signal with an angular frequency of xcfx89c to obtain the phase term of the received signal. The controller feedback controls a driving condition for the coherent light source so that the amplitude of the phase term extracted by the phase extractor matches to a predetermined setting.
Second, the present invention is provided with a wavelength-stabilized light source apparatus, which comprises a reference light source for emitting a coherent reference light with a constant wavelength; a coherent light source of which wavelength is variably controlled by a driver; a beam splitter for splitting the output light from the coherent light source to obtain a part thereof as a control light; a interference optical system for receiving the control light split by the beam splitter and the reference light from the reference light source via different optical paths, splitting the control and reference lights respectively in two, then giving a predetermined optical path length difference respectively to the resultant two control and reference lights and synthesizing them to generate two interference fringes; a modulator for giving the same variation respectively to the optical path length differences of the reference and control lights in the interference optical system to modulate the two interference fringes obtained with respect to the reference and control lights; a pair of photoreceptive devices for receiving the two interference fringes with respect to the reference and control lights output from the interference optical system; a pair of demodulators for detecting degrees of modulations of received signals obtained with respect to the reference and control lights from the pair of photoreceptive devices; and a controller for feedback controlling a driving condition for the driver so that a ratio between the outputs from the pair of demodulators matches to a predetermined setting.
In the second wavelength-stabilized light source apparatus, in combinations with the reference light source, the control light or a part of the output light from the coherent light source and the reference light are introduced into the interference optical system to modulate respective optical path length differences, resulting in the two interference fringes. Using the dependency of the degrees of modulations of the received signals with respect to the reference and control lights on their wavelengths, the wavelength stabilization is performed by feedback controlling the driver so that the ratio between the demodulated outputs matches to a predetermined setting. Thus, by the designation from the external, on the basis of the relation between the wavelength of the reference light and the output wavelength of the coherent light source, the latter can be stabilized at a high speed with a high precision.
In the second wavelength-stabilized light source apparatus, there are several modes to modulate the received signals by giving a variation to the optical path length differences for the control and reference lights as follows:
(a) In a first mode, the variation given to the optical path length differences for the control and reference lights is an additional or reductive displacement at a constant velocity to amplitude-modulate the two interference fringes obtained with respect to the reference and control lights in according with wavelengths thereof, respectively.
In this case, the demodulators detect frequencies, f1 and f2, of intensity variations of the amplitude-modulated received signals obtained with respect to the reference and control lights. The controller feedback control the driver for the light source so that a ratio between the frequencies f1 and f2 matches to a predetermined setting.
(b) In a second mode, the variation given to the optical path length differences for the control and reference lights is a vibration in the form of a sinusoidal wave with a predetermined amplitude of d and an angular frequency to phase-modulate the received lights obtained with respect to the reference and control lights in accordance with wavelengths thereof, respectively.
In this case, the demodulators PM-demodulate the phase-modulated received signals with respect to the reference and control lights to detect the phase terms, "psgr"1=(2xcfx80d/xcex1)+xcfx861 and "psgr"2=(2xcfx80d/xcex2)+xcfx862 (where xcex1, xcex2 denote wavelengths of the reference and control lights, and xcfx861, xcfx862 initial phase magnitudes), and extract the amplitudes of the phase terms. The controller feedback controls the driver for the light source so that a ratio, xcex2/xcex1, between the phase terms matches to a predetermined setting.
(c) In a third mode, imparting a vibration in the form of a sinusoidal wave with a predetermined amplitude of d and an angular frequency of xcfx89 both on the optical path length differences of the reference and control lights; and simultaneously from acoustic optical modulators interposed in the optical paths, superimposing an interfered beat signal with an angular frequency of xcfx89c both on the received signals of the reference and control lights to frequency-modulate the received signals obtained with respect to the reference and control lights in accordance with wavelengths thereof.
In this case, the demodulators FM-demodulate the frequency-modulated received signals obtained with respect to the reference and control lights using a carrier angular frequency of xcfx89c to detect the phase terms, "psgr"1=(2xcfx80d/xcex1)+xcfx861 and 1042=(2xcfx80d/xcex2)+xcfx862 (where xcex1, xcex2 denote wavelengths of the reference and control lights, and xcfx861, xcfx862 initial phase magnitudes), and extract amplitudes of these phase terms. The controller feedback controls the driver for the light source so that a ratio xcex2/xcex1, between the phase terms matches to a predetermined setting.
The method of imparting the sinusoidal wave vibration both on the optical path length differences of the reference and control lights may include a method of mechanically imparting a signal on a mirror and the like in the interference optical system using a piezoelectric device and the like. It may also include a method of electrical modulation using an electric optical modulator.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.